1. Field of the Invention
The present invention relates generally to waveform generators.
2. Description of the Related Art
The generation of sinusoidal waveforms (i.e., sine and cosine signals) is a fundamental process in an enormous array of electronic systems (e.g., communication systems, display systems, and radar systems). Accordingly, a substantial number of sinusoidal generators have been developed such as traditional feedback oscillators that realize sinusoidal waveforms with the aid of feedback structures (e.g., RC phase-shift networks, LC resonant circuits, tuning forks and crystals).
Although feedback oscillators can generate low-harmonic, highly-stable sinusoidal signals, they generally fail to provide the wide tuning range that is obtained, for example, by generators that use triangle-driven signal shapers. An exemplary one of these generators provides a triangle waveform (e.g., with a digital-to-analog converter (DAC)) and utilizes the nonlinear logarithmic relationship between base-emitter voltage Vbe and collector current Ic as a logarithmic shaper to smooth the triangle waveform into a corresponding sinusoidal waveform.
Another exemplary wide-range generator shapes a triangle waveform with a breakpoint shaper that sequentially turns on and off an array of diodes that selectively couple a corresponding array of resistors into the waveform's path.
Digital processes are used to realize other wide-range waveform generators. For example, a phase accumulator (or an up/down counter) provides a first stream of successive binary words that are spaced by a designated phase step. The first stream can be passed through a lookup table which converts the first stream into a second stream that defines a sinusoidal waveform.
The actual waveform is then generated by a DAC which receives the second stream of digital words. Alternatively, the first stream of digital words can be passed directly to a DAC to provide a triangle waveform which is then shaped by wave shapers (e.g., as described above) into sinusoidal waveforms.
In another type of digital waveform generator, a CORDIC algorithm is processed to determine (e.g., in a microprocessor) sine and cosine values of selected angles as the coordinates along x and y axes of a successively rotated phasor.
Although various structures and methods thus exist for wide-range generation of sinusoidal waveforms, they typically include requirements (e.g., signal shapers require closely-controlled amplitudes, lookup tables require large portions of chip area and algorithms generally require significant calculation time) that are not compatible with simple, low-cost, wide-range generation of sinusoidal waveforms.